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NASA-backed scientists have used a once-in-a-lifetime chance encounter of objects in our galaxy to spot the first signs of an exomoon - a moon outside our Solar System.

Because the stars literally had to be aligned to spot the rocky body in space, researchers won't be able to spot it again, but finding it once could help scientists to locate others.

"We won't have a chance to observe the exomoon candidate again," said David Bennett of the University of Notre Dame, lead author of a new paper on the findings appearing in the Astrophysical Journal. "But we can expect more unexpected finds like this."

Exomoons are exciting discoveries because they could help scientists to learn more about exoplanets - worlds lying outside our Solar System - through a technique called gravitational microlensing.

Possible exomoon or potential exoplanet?(Credit: NASA/JPL-Caltech)

This study was led by the Japan-New Zealand-American Microlensing Observations in Astrophysics (MOA) and the Probing Lensing Anomalies NETwork (PLANET) programmes, using telescopes in New Zealand and Tasmania. Both projects use gravitational microlensing to see beyond chance alignments of the stars.

When a star passes between the Earth and a more distant celestial body, the closer star can act like a magnifying glass, focusing and brightening the light of the stars beyond. If the nearer star has a planet orbiting it, the planet can act as a second lens to brighten the light even more. Scientists can then use the brightening events, which can last around a month, to figure out the mass of the foreground star relative to its planet.

In the same way, the foreground object could be a planet instead of a star and scientists could use the event to measure the mass of the planet relative to its orbiting companion - a moon.

In this study, the researchers aren't sure what the foreground object is. The ratio of the larger body to its orbiting companion is 2,000 to one, so the pair could be a small, faint star circled by a planet around 18 times the mass of Earth - or it could be a planet more massive than Jupiter orbited by its own moon.

"One possibility is for the lensing system to be a planet and its moon, which if true, would be a spectacular discovery of a totally new type of system," said Wes Traub, the chief scientist for NASA's Exoplanet Exploration Program office at the agency's Jet Propulsion Laboratory, who was not involved in the study.

"The researchers' models point to the moon solution, but if you simply look at what scenario is more likely in nature, the star solution wins."

Although less likely, the planet-moon combo is not without precedent. Exoplanet-hunting surveys have already found Jupiter-sized worlds out roaming the stars alone, probably because they were kicked out of forming planetary systems by the gravitational effects of getting too close to other planets or stars.

To solve the puzzle, the researchers would need to figure out how far away the pair of celestial bodies is - a lower-mass duo closer to Earth would look the same as a more massive pair further away. But once a brightening event is over, it's very difficult to take additional measurements of the lensing system, meaning this exomoon's true identity will remain a mystery.

In the future, however, it might be possible to work out distances during brightening events using the parallax technique along with gravitational lensing. NASA's Spitzer and Kepler space telescopes are both great tools for this technique because they're far enough away from Earth.

If you hold out your finger and close one eye after the other, your finger seems to jump back and forth - scientists can use that same phenomenon to help identify object measurements using two telescopes that are really far away from each other, such as one in space and one back home on Earth. Combined with the lensing technique, the parallax effect changes how a telescope will view the resulting magnification.

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